Many types of transmitting and receiving systems use phased array antennas. It is often desirable to change the direction of radiation of such a phased array antenna. This might be the case in a phased array antenna used for shipboard or airborne radar, or for a phased array antenna located on an on Earth satellite for changing its "footprint" on the Earth to match the shape of a particular land mass.
Control of the preferred direction of radiation of a phased array antenna is accomplished by selecting the phase of the signals applied to the antenna elements in a manner well known in the antenna arts. Similarly, due to the reciprocity of transmission and reception as also well known in the antenna arts, the direction from which a phased array receiving antenna receives signals can be established by the phasing of the signals applied from the antenna elements to a common summing point. In those cases in which changes in the direction of radiation of a phased array antenna is desired during operation, it is common to employ variable phase shifters located in the various feed structures of the individual antenna elements and to control those phase shifters to achieve the desired result. Analog phase shifters using variable reactance elements such as variable capacitance diodes can be used to achieve the desired phase shifts. However, such analog phase shifters are not well adapted to highly accurate remote control such as is necessary in many applications. For highly accurate remote control phase shifts, switching type phase shifters are normally used. Switching type phase shifters employ controllable switches to couple various predetermined lengths of transmission line into and out of circuit. As known, the introduction of an incremental length of transmission line into a transmission line causes a differential phase shift of the signal passing therethrough.
Modern designs use semiconductor switching elements for switching type phase shifters. The diodes are driven with a forward current to reduce their ON (conductive) condition resistance to allow the signal being phase shifted to flow therethrough, and are biased with a reverse bias voltage to turn them OFF (nonconductive) to prevent the flow of signal therethrough. It has been found that for many high power applications such as for phased array radar, PIN type diodes are preferable to PN junction diodes.
No matter which type of diode is used, a driver circuit is required for providing the bias for maintaining the diode in the ON or in the OFF condition. When high power is to be handled, the signal currents and voltages may be large. Consequently, operation of the diode as a switch requires relatively large bias currents and voltages. When the bias source applies a large forward current to a diode to switch it into the ON condition, the junction region becomes flooded with excess charge carriers. When a reverse voltage is thereafter applied to the diode to switch it to the OFF condition, these excess charge carriers provide continued conduction through the junction region, and therefore the diode does not immediately turn off as to the signal being phase shifted, even though no forward bias current is applied. Many applications of phased array radars require that the direction of radiation (or reception) be changed quickly, and this in turn requires high speed operation of the phase shifters. That portion of the switch driver circuit of a phase shifter which provides the reverse voltage bias must be capable of carrying a high current in order to quickly extract the excess charge carriers and thereby render the diode nonconductive. U.S. Pat. No. 4,005,361 issued Jan. 25, 1977, to Lerner describes such a driver circuit in which an active switch provides forward bias current to the switching diode and to a resistor. When the active switch turns OFF, the resistor provides the reverse bias voltage. For the diode to turn OFF quickly, the resistor must have a low resistance. This low resistance is disadvantageous in that a large amount of power is dissipated therein during the ON time of the active switch. The high currents and voltages which must be handled by the driver circuit for switching diodes used in the phase shifters of high power equipment such as radars tend to cause failure of the switching diodes and of driver transistors.
A large phased array antenna may have more than 4000 antenna elements. Each antenna element includes a phase shifter which musst be capable of shifting phase in relatively small increments. The number of elements in a phase shifter for an antenna array is minimized by causing the electrical length of the elements of the phase shifter to follow a binary progression. For example, an increment size or phase granularity of 22.5.degree. may be accomplished by a phase shifter including switchable elements having electrical lengths of 180.degree., 90.degree., 45.degree. and 22.5.degree.. Any phase shift ranging from 0.degree. to 337.5.degree. in 22.5.degree. increments may be obtained by combinations of these various elements. Each of the four elements may be switched ON or OFF, and corresponds to one bit. Thus, a four bit phase shifter is capable of 22.5.degree. increment size or granularity. It is not uncommon to have each antenna element associated with four or more phase shifter elements. It can be seen that a large antenna array with a fine granularity may require a very large number of phase shift elements, each of which must be operated by a driver. Because of the large number of phase shifter elements, a failure of one or even several drivers or their associated diodes may go unnoticed, as the amount of degradation of the overall system is small. Even if the degradation is noticed, it is very difficult to locate a defective element in a large array. The problem of locating defective elements is even worse for an element which has not grossly failed but is simply degraded. For example, a degradation of the switching time of one or several diodes may cause a degradation in the rate of slew of the pointing direction of the antenna which is not perceptible to humans but which may be significant in terms of the mission to be accomplished. Location of a phase shifter element having degraded switching speed is extremely difficult in a large array. Consequently, there is a need for automatic self-test of the condition of drivers and diodes in a large array. The aforementioned Lerner patent describes a circuit for monitoring the accuracy of the phase shift produced in a radio frequency power distribution network including diodes. In the Lerner arrangement, analog diode voltages are multiplexed under the control of the digital computer and applied to comparators for comparison with reference voltages. The resulting comparison produces a digital signal which is applied to a digital computer for evaluation and generation of an alarm signal. In a large phased array antenna system, especially one handling high power signals, it is very undesirable to multiplex analog signals or to run analog signals over conductors of any significant length, because of the mutual coupling and interference problems which may arise. Furthermore, in systems having high slew rates which require fast switching speeds, it may not be possible to dynamically measure the condition of the diodes because of the delays introduced by the long line lengths.